1. Field of the Invention
The present invention relates to a power conversion device such as a sputtering power supply device used in a DC (direct current) sputtering apparatus, and in particular relates to a power conversion device which is suitable for performing power conversion for a commercial AC (alternating current) high voltage power supply system which generates a voltage within the range of AC 400V to 480V.
2. Description of the Related Art
A DC sputtering apparatus introduces inert gas such as argon gas into a vacuum chamber, applies a negative polarity voltage of several hundred volts to a target electrode, and causes plasma discharge to positively ionize the inert gas. Such a DC sputtering apparatus is used as a thin film deposition apparatus which accelerates positive ions and strikes the accelerated positive ions on a target surface to evaporate a target material, and which deposits vapor of the target material on a surface of a semiconductor or on a substrate such as an optical disc to form a thin film of the target material. The rated power of a sputtering power supply device which causes plasma discharge in a vacuum or in insulating gas by using a relatively low voltage falls within a wide range of several kW to several tens kW.
Although not shown in the drawings, such a sputtering power supply device is generally provided with a rectifying circuit which converts a commercial AC voltage into a DC voltage, a single inverter circuit which converts the DC voltage output from the rectifying circuit into a high frequency AC voltage, and another rectifying circuit. The inverter circuit includes a transformer which boosts the high frequency AC voltage, and another rectifying circuit converts an AC high voltage across a secondary winding of the transformer into a predetermined high DC voltage (see, for example, to the description on page 6 to page 8 and FIG. 1 of Japanese Unexamined Patent Application, First Publication No. 2001-145371).
An AC voltage input to the aforementioned sputtering power supply device generally falls within the range of AC 200V to 220V. However, an AC voltage input to a high capacity sputtering DC power supply device falls within the range of AC 400V to 480V (hereinafter referred to as “commercial AC power supply high voltage”). When an input AC voltage falls within the range of AC 200V to 220V, the rectified voltage output from the rectifying circuit is about 300V. Therefore, it is possible to use FETs being inexpensive and having a low on-resistance value and a low withstand voltage (e.g., the withstand voltage of 500V) as semiconductor switching devices provided in the inverter circuit. However, when an input voltage is the commercial AC power supply high voltage which falls within the range of AC 400V to 480V, the maximum of the rectified voltage output from the rectifying circuit becomes a DC high voltage of approximately 620V to 744V including the fluctuation thereof. As a result, the withstand voltages of the semiconductor switching devices provided in the inverter circuit should be at least about 1000V. Accordingly, it is necessary to use FETs or IGBTs having a withstand voltage of 1000V or more as the semiconductor switching devices.
As stated above, FETs or IGBTs having a withstand voltage of 1000V or more are used in a 400V system of the commercial AC power supply high voltage whose input voltage falls within the range of 400V to 480V. However, FETs having high withstand voltages generally have high on-resistance values and bring about high power loss. In contrast, IGBTs having high withstand voltages and being capable of supplying high current are readily available, but such type of IGBT has low switching speed and it is hard to operate the IGBT at a high frequency. As a result, the size of a power conversion device becomes large and it becomes difficult to reduce the voltages of ripples.